Instability of gene-specific microsatellite and minisatellite repetitive sequences, leading to increase in length of the repetitive sequences in the satellite, is associated with about 35 human genetic disorders. Instability of trinucleotide repeats is for instance found in genes causing X-linked spinal and bulbar muscular atrophy (SBMA), myotonic dystrophy type 1 (DM1), fragile X syndrome (FRAX genes A, E, F), Huntington's disease (HD) and several spinocerebellar ataxias (SCA gene family). Unstable repeats are found in coding regions of genes, such as the Huntington's disease gene, whereby the phenotype of the disorder is brought about by alteration of protein function and/or protein folding. Unstable repeat units are also found in untranslated regions, such as in myotonic dystrophy type 1 (DM1) in the 3′ UTR or in intronic sequences such as in myotonic dystrophy type 2 (DM2). The normal number of repeats is around 5 to 37 for DMPK, but increases to premutation and full disease state two to ten fold or more, to 50, 100 and sometimes 1000 or more repeat units. For DM2/ZNF9 increases to 10,000 or more repeats have been reported. (Cleary and Pearson, Cytogenet. Genome Res. 100: 25-55, 2003).
The causative gene for Huntington's disease, HD, is located on chromosome 4. Huntington's disease is inherited in an autosomal dominant fashion. When the gene has more than 35 CAG trinucleotide repeats coding for a polyglutamine stretch, the number of repeats can expand in successive generations. Because of the progressive increase in length of the repeats, the disease tends to increase in severity and presents at an earlier age in successive generations, a process called anticipation. The product of the HD gene is the 348 kDa cytoplasmic protein huntingtin. Huntingtin has a characteristic sequence of fewer than 40 glutamine amino acid residues in the normal form; the mutated huntingtin causing the disease has more than 40 residues. The continuous expression of mutant huntingtin molecules in neuronal cells results in the formation of large protein deposits which eventually give rise to cell death, especially in the frontal lobes and the basal ganglia (mainly in the caudate nucleus). The severity of the disease is generally proportional to the number of extra residues.
DM1 is the most common muscular dystrophy in adults and is an inherited, progressive, degenerative, multisystemic disorder of predominantly skeletal muscle, heart and brain. DM1 is caused by expansion of an unstable trinucleotide (CTG)n repeat in the 3′ untranslated region of the DMPK gene (myotonic dystrophy protein kinase) on human chromosome 19q (Brook et al, Cell, 1992). Type 2 myotonic dystrophy (DM2) is caused by a CCTG expansion in intron 1 of the ZNF9 gene, (Liguori et al, Science 2001). In the case of myotonic dystrophy type 1, the nuclear-cytoplasmic export of DMPK transcripts is blocked by the increased length of the repeats, which form hairpin-like secondary structures that accumulate in nuclear foci. DMPK transcripts bearing a long (CUG)n tract can form hairpin-like structures that bind proteins of the muscleblind family and subsequently aggregate in ribonuclear foci in the nucleus. These nuclear inclusions are thought to sequester muscleblind proteins, and potentially other factors, which then become limiting to the cell. In DM2, accumulation of ZNF9 RNA carrying the (CCUG)n expanded repeat form similar foci. Since muscleblind proteins are splicing factors, their depletion results in a dramatic rearrangement in splicing of other transcripts. Transcripts of many genes consequently become aberrantly spliced, for instance by inclusion of fetal exons, or exclusion of exons, resulting in non-functional proteins and impaired cell function.
The observations and new insights above have led to the understanding that unstable repeat diseases, such as myotonic dystrophy type 1, Huntington's disease and others can be treated by removing, either fully or at least in part, the aberrant transcript that causes the disease. For DM1, the aberrant transcript that accumulates in the nucleus could be down regulated or fully removed. Even relatively small reductions of the aberrant transcript could release substantial and possibly sufficient amounts of sequestered cellular factors and thereby help to restore normal RNA processing and cellular metabolism for DM (Kanadia et al., PNAS 2006). In the case of HD, a reduction in the accumulation of huntingtin protein deposits in the cells of an HD patient can ameliorate the symptoms of the disease.
A few attempts have been made to design methods of treatment and medicaments for unstable repeat disease myotonic dystrophy type 1 using antisense nucleic acids, RNA interference or ribozymes.
(i) Langlois et al. (Molecular Therapy, Vol. 7 No. 5, 2003) designed a ribozyme capable of cleaving DMPK mRNA. The hammerhead ribozyme is provided with a stretch RNA complementary to the 3′ UTR of DMPK just before the CUG repeat. In vivo, vector transcribed ribozyme was capable of cleaving and diminishing in transfected cells both the expanded CUG repeat containing mRNA as well as the normal mRNA species with 63 and 50% respectively. Hence, also the normal transcript is gravely affected by this approach and the affected mRNA species with expanded repeats are not specifically targeted.
(ii) Another approach was taken by Langlois et al., (Journal Biological Chemistry, vol 280, no. 17, 2005) using RNA interference. A lentivirus-delivered short-hairpin RNA (shRNA) was introduced in DM1 myoblasts and demonstrated to down regulate nuclear retained mutant DMPK mRNAs. Four shRNA molecules were tested, two were complementary against coding regions of DMPK, one against a unique sequence in the 3′ UTR and one negative control with an irrelevant sequence. The first two shRNAs were capable of down regulating the mutant DMPK transcript with the amplified repeat to about 50%, but even more effective in down regulating the cytoplasmic wildtype transcript to about 30% or less. Equivalent synthetic siRNA delivered by cationic lipids was ineffective. The shRNA directed at the 3′ UTR sequence proved to be ineffective for both transcripts. Hence, also this approach is not targeted selectively to the expanded repeat mRNA species.
(iii) A third approach by Furling et al. (Gene Therapy, Vol. 10, p 795-802, 2003) used a recombinant retrovirus expressing a 149-bp long antisense RNA to inhibit DMPK mRNA levels in human DM1 myoblasts. A retrovirus was designed to provide DM1 cells with the 149 bp long antisense RNA complementary to a 39 bp-long (CUG)13 repeat and a 110 bp region following the repeat to increase specificity. This method yielded a decrease in mutated (repeat expanded) DMPK transcript of 80%, compared to a 50% reduction in the wild type DMPK transcript and restoration of differentiation and functional characteristics in infected DM1 myoblasts. Hence, also this approach is not targeted selectively to the expanded repeat mRNA species, it depends on a very long antisense RNA and can only be used in combination with recombinant viral delivery techniques.